In U.S. Pat. Nos. 2,887,496 and 2,930,801 there is disclosed processes for the preparation of methacrylic acid by (1) the oxidation of methacrolein in aqueous sodium hydroxide solution in the presence of a silver catalyst to produce the sodium salt of the methacrylic acid, i.e., an aqueous sodium methacrylate solution, which solution is (2) treated with a strong acid such as sulfuric to liberate the methacrylic acid from the sodium salt. The free acid can be separated by appropriate methods such as ether extraction.
The present invention is directed to an effective method of recovering the methacrylic acid from an aqueous sodium methacrylate solution produced, for example, by the above described processes by contacting the aqueous sodium methacrylate solution after separation of the silver catalyst, for example, by filtration, at a suitable temperature with a water immiscible alkane, cycloalkane, alkyl benzene, or carboxylic acid ester solvent to extract the methacrylic acid formed in the presence of a high carbon dioxide pressure to acidify the aqueous solution. Liquid carbon dioxide at its equilibrium vapor pressure may also be employed at temperatures of from about 0.degree. C. to 30.degree. C. to react with the sodium methacrylate and extract the methacrylic acid formed with the precipitation of sodium bicarbonate.
Since the solubility of sodium bicarbonate in water is relatively high, an essential feature of the recovery process of the invention is the use of high CO.sub.2 pressures in conjunction with a water immiscible organic solvent to (1) lower the solubility of the precipitated sodium bicarbonate (formed by the reaction) in water and (2) provide and allow for an effective extraction of the methacrylic acid formed respectively thus, shifting the equilibrium reaction to the right to completion and high acid recovery in accordance with the following: EQU CH.sub.2 .dbd.C(CH.sub.3)COONa+CO.sub.2 +H.sub.2 O.sup.solvent CH.sub.2 .dbd.C(CH.sub.3)COOH+NaHCO.sub.3 ps
A free methacrylic acid-containing organic solvent phase is formed along with an aqueous phase and a precipitated sodium bicarbonate phase. The phases may be separated by any conventional method, e.g., decantation, filtration and centrifugation. The methacrylic acid may be separated and recovered from the organic solvent phase by for example distillation; the organic solvent being recovered for recycle. Liquid carbon dioxide at its equilibrium vapor pressure at temperatures of from about 0.degree. C. to 30.degree. C. may also be employed in the reaction, without additional solvent, to also act as the water immiscible organic solvent phase for extraction of the methacrylic acid as formed. After separation of the aqueous and precipitated sodium bicarbonate phases, the methacrylic acid may be recovered from the liquid CO.sub.2 -containing methacrylic acid phase by lowering the pressure to form CO.sub.2 gas which may be released leaving the methacrylic acid product. Aqueous sodium methacrylate solutions are not neutralized to any significant extent by carbon dioxide alone due to an unfavorable equilibrium except as high CO.sub.2 pressures and in the presence of an organic phase.
U.S. Pat. No. 3,839,437, describes a process of preparing methacrylic acid by employing calcium hydroxide to form the metal salt of the acid, calcium methacrylate, and neutralization with CO.sub.2 at low pressure to give the acid and the insoluble calcium carbonate in the presence of a water immiscible organic solvent, such as an ether, to extract the methacrylic acid from the aqueous phase. Continuous passage of carbon dioxide with ether extraction gave only 10.6 and 41.3 percent methacrylic acid recovery in 1 and 4 hours respectively.
British Pat. No. 1,055,229 discloses a process for the use of carbon dioxide to neutralize calcium methacrylate formed by oxidizing methacrolein in an aqueous medium of an alkaline earth metal hydroxide using cupric oxide or hydroxide and a noble metal, noble metal oxide or hydroxide. The methacrylic acid is recovered by acidification with a mineral acid.
French Pat. No. 1,503,918 discloses a process for the silver catalyzed oxidation of methacrolein to prepare an aqueous solution of sodium methacrylate.
U.S. Pat. No. 3,579,572 describes a process in which carbon dioxide is employed to treat an aqueous lithium or magnesium terephthalate under pressure to precipitate terephthalic acid.
Many important commercial applications have been developed for the methacrylic acid product of this invention, significantly, for the preparation of acrylate type polymers and resins.